Bryan Pendleton points to some interesting Commentary discussing Nehalem Power step down, and generally criticising the BIOS, OS and CPU for stepping down the CPU voltage. I don't agree with all the criticisms, because in a large datacentre, electricity is the biggest monthly operating expense. By cutting down on CPU voltage you make a massive saving in power for less performance impact.
However, in the use cases discussed -real time database queries- it doesn't seem to work. The problem is even once the BIOS puts the OS in charge, the OS is slow to react to bursty load. Better to put the decisions in the hardware itself. IBM did this with some laptop HDDs a few years back, spinning up and down based on the length of the queue. But it didn't take off as nobody else adopted it, all laptop vendor want the ability to switch HDD suppliers, and users kept complaining about the way the noise their disk was making kept changing.